Submitted since comments can get deleted, and it's been almost 2 years.
Joe White identified the book as "Below the Root" in a couple of comments.
Sounds like a book I read once. I remember the main character, a girl,
saying to her brother at one point that she'd like to "dead him"
(using "dead" as a verb because they had no word for "kill") and her
family reacting in complete shock. And there was a society of
ground-dwellers too, and they did remember the word for "kill", and
the MC met a ground-dwelling girl around her age... any of this
familiar? (Unfortunately I read the book in school, also around 25-30
years ago, and have long since forgotten title or author.)
Just checked out the e-book from the library, and while I didn't
remember all the details correctly, the book I was thinking of is
definitely "Below the Root", the first book in Zilpha Keatley Snyder's
"Green Sky" trilogy.
The questioner Laura accepted the ID in a comment.
That is it! Below the Root. Zilpha Keatley Snyder is such a great
author. Thanks so much!